1. Field of the Invention
The present invention relates to an AC servomotor usable as an actuator of various industrial machines, such as robots, and, more particularly, to a small AC servomotor providing a high output.
Even more particularly, the present invention relates to an AC servomotor using a polar anisotropic magnet in a rotor. Still even more particularly, the present invention relates to an AC servomotor which is controllable with high precision as a result of reducing cogging torque produced between a magnet and a stator-side iron core.
2. Description of the Related Art
In general, a servomotor is used as an actuator of an industrial machine because a servomotor is easy to handle, is small, provides high torque, and is highly responsive. In particular, since an AC servomotor is brushless and does not require maintenance, it can be used as, for example, a joint actuator of an automatic machine which is desired to move in a working space without human intervention, such as a walking robot. In an AC servomotor, a permanent magnet is disposed at the rotor side, while a coil is disposed at the stator side. The AC servomotor generates a torque based on a sinusoidal magnetic flux distribution and by a sinusoidal electrical current.
For example, a small AC servoactuator applicable to a legged walking robot is disclosed in Japanese Unexamined Patent Publication No. 11-33386 which has already been assigned to the applicant. The servoactuator disclosed in the specification is a type directly connected to a gear and incorporates in a motor unit a servo control system formed into a one-chip system.
In particular, an AC servomotor used in an autonomous walking robot is required to be small and to provide high output, high speed, and a stable control operation. The stator side is constructed so that the wires are wound close together using a slit-core method, and a magnet having a high magnetic flux density is used at the rotor side in order to make it possible to reduce the size of the servomotor and to make it provide high output. (This is already well known by those skilled in the art.)
Depending on whether or not a magnetic field is applied when molding magnetic powder inside a mold, a magnet is classified as either an isotropic or an anisotropic magnet. More specifically, when a magnetic field is not applied during molding, the magnetic field inside the magnet becomes uniform or isotropic. In contrast, when a magnetic field is applied during molding, the magnetic field inside the magnet is oriented, that is, becomes anisotropic, thereby making it possible to obtain a high magnetic flux even if the magnet is small.
By the orientation of the magnetic field formed inside the magnet, an anisotropic magnet is classified as either a radial anisotropic magnet or a polar anisotropic magnet. FIGS. 1 and 2 schematically illustrate the orientation of the magnetic field inside an annular radial anisotropic magnet in a cross section thereof, and the orientation of the magnetic field inside an annular polar anisotropic magnet in a cross section thereof, respectively. In particular, since the polar anisotropic magnet has a high magnetic flux, it is an excellent magnet from the viewpoint of its high output. However, since the polar anisotropic magnet is magnetized in accordance with its magnetic path during molding and magnetization, one cannot set N/S poles on desired locations of the ring.
When a magnet having a high magnetic flux is used to achieve higher output, a torque ripple, called a xe2x80x9ccogging torque,xe2x80x9d which is generated between the magnet and the stator-side iron core increases. Unless this torque ripple is reduced, the servomotor cannot be controlled with high precision, thereby making it impossible to realize higher performance which is required of an actuator.
It is known that cogging torque does not depend upon the windings of the stator, but that it is generated by changes in the magnetic flux that occurs as the magnetized magnet rotates with respect to a slot of the iron core of the stator.
In order to reduce cogging torque, a method of skewing (that is, tilting) the magnetic poles of the magnet is used. FIG. 3 illustrates a rotor in which a radial anisotropic magnet has been subjected to a skew magnetization operation. In the case of a radial anisotropic magnet, when such a magnet used in the rotor is magnetized, it is possible to skew each of the magnetic poles of the magnet as shown in FIG. 3.
However, in the case of the above-described polar anisotropic magnet, the direction of magnetization is determined by the formation of the magnetic field when the magnet is formed by molding, thereby making it difficult to subject it to a skew magnetization operation during the magnetization. Therefore, when a polar anisotropic magnet is used in order for the servomotor to provide high output, the problem of cogging torque remains.
For example, Japanese Unexamined Patent Application Publication No. 8-340652 discloses the reduction of cogging torque in an AC servomotor including an annular polar anisotropic magnet used in a rotor. More specifically, in the AC servomotor disclosed in the document, the annular polar anisotropic magnet used in the rotor is split into two or more magnets in an the axial line direction thereof, and the magnetic poles of each of the magnets are shifted by a predetermined skew angle xcex8 and fixed in order to reduce cogging torque apparently by an effect similar to that obtained when a skew magnetization operation is carried out.
The skew angle xcex8 should be set at an optimal value which causes the cogging torque to be a minimum. However, Japanese Unexamined Patent Application Publication No. 8-340652 does not particularly discuss at what value the skew angle xcex8 should be set.
In the case of a radial anisotropic magnet, the angle which causes the cogging torque to be cancelled (that is, which causes opposite phases to be produced) can be used as the skew angle xcex8 which is determined based on the number of torque ripples per rotation of the rotor determined by the number of magnetic poles of the annular magnet used in the rotor and the number of slots of the stator iron core.
For example, in the case where a magnet is a four-pole magnetization type magnet and has six slots in the iron core of the stator, when the rotor is constructed using one layer of an annular polar anisotropic magnet, twelve torque ripples are produced, with twelve being the least common multiple of the number of magnetic poles and the number of slots. In other words, the period of the cogging torque is 30 degrees. Therefore, by performing magnetization after the skew angle xcex8 has been set at 15 degrees, which corresponds to half the period of the cogging torque, the torque ripples produced based on the number of slots in the iron core and by the magnet are such as to have opposite phases, and, thus, cancel each other, thereby minimizing cogging torque.
However, in a servomotor such as that disclosed in Japanese Unexamined Patent Application Publication No. 8-340652 in which an annular polar anisotropic magnet used in a rotor is split into two or more magnets in the axial line direction thereof, and the poles of the polar anisotropic magnets are shifted by a skew angle xcex8, the upper and lower annular polar anisotropic magnets magnetically interfere with each other at the boundary therebetween. This magnetic interference prevents the skew angle xcex8 (which corresponds to half the period of the cogging torque) obtained by the above-described method from matching the optical angle which causes the cogging torques to cancel each other.
Accordingly, it is an object of the present invention to provide an excellent AC servomotor usuable as an actuator of various industrial machines including robots.
It is another object of the present invention to provide an excellent AC servomotor which is small and which provides high output.
It is still another object of the present invention to provide an excellent AC servomotor using a polar anisotropic magnet in a rotor.
It is still another object of the present invention to provide an excellent AC servomotor which can be controlled with high precision as a result of reducing cogging torque generated between a magnet and an iron core of a stator.
To these ends, according to the present invention, there is provided an AC servomotor using an annular polar anisotropic magnet in a rotor. In the AC servomotor, the annular polar anisotropic magnet is split into two or more annular polar anisotropic magnets in an axial line direction thereof. Magnetic poles of the corresponding split annular polar anisotropic magnets are disposed so as to be shifted by a predetermined angle xcex8xe2x80x2 which is greater than a skew angle xcex8 determined based on the number of torque ripples per rotation of the rotor determined by the number of magnetic poles of the annular magnet at the rotor side and the number of slots in a stator-side iron core.
In one form of the invention, the skew angle xcex8 is, for example, equal to half the period of a cogging torque determined based on the number of torque ripples per rotation of the rotor determined by the number of magnetic poles of the annular magnet and the number of slots in the stator-side iron core. Ordinarily, the number of torque ripples generated per rotation corresponds to the least common multiple of the number of magnetic poles and the number of slots.
In another form of the invention, the predetermined angle xcex8xe2x80x2 is an angle obtained by adding to the skew angle xcex8 a value which takes into consideration magnetic interference between the split annular polar anisotropic magnets.
In still another form of the invention, the predetermined angle xcex8xe2x80x2 is, for example, approximately {fraction (4/3)} times the skew angle xcex8 which corresponds to half the period of a cogging torque determined based on the number of torque ripples per rotation of the rotor determined by the number of magnetic poles of the annular magnet and the number of slots in the stator-side iron core.
According to the AC servomotor of the present invention, even when an annular polar anisotropic magnet is used in the rotor, the cogging torque can be restricted to a minimum. More specifically, according to the present invention, it is possible to realize a servomotor having a small cogging torque and providing a high output.
As a result of restricting the cogging torque to a minimum, it is possible to provide an AC servomotor which is controllable with high precision. Such a servomotor which can provide high output and which is controllable with high precision can be used as a joint actuator in legged walking robots and other types of articulated robots. In particular, such a servomotor can contribute to the increasing of the performance of small robots.